NSA's information gathering practices have been further detailed in court papers revealed by The Guardian. While the agency has continued to reiterate that it doesn't collect its data indiscriminately, the leaked papers detail several loopholes that allow it to gather data from both American and foreign origins without the need for a warrant. If you use data encryption or other privacy tools, your communications are likely to receive extra attention, and the agency can indefinitely keep any information assembled for "crypto-analytic, traffic analysis or signal exploitation purposes" -- in short, if the NSA believes may be relevant in the future.
One reason to hold onto said files could simply be the fact that the data is encrypted and NSA wants to be able to analyze its protection. The security agency can also give the FBI and other government organizations any data if it contains a significant amount of foreign intelligence, or information about a crime that has (or will be) committed. Any data that's "inadvertently acquired" through the NSA's methods -- and could potentially contain details of US citizens -- can be held for up to five years before it has to be deleted. The Guardian's uploaded the leaked papers in full -- hit the source links for more.